A complicated underground electrical characteristic distribution is conventionally measured by providing a plurality of current transmission points and potential measurement points on the surface of the ground or in the ground, selecting various combination of pairs of current point and potential measurement point, providing current transmission from the current point, observing potential response at the potential measurement point. However, the conventional method avoids simultaneous current transmission from multiple points and merely transmits a current from only one transmission point during one measurement process and therefore a current is not transmitted from multi-points simultaneously (see Patent Documents 1 and 2 and Non-Patent Document 1).
Thus, time required for measurement needs to have at least the value of time required for one measurement process multiplied by the number of transmission points and, accordingly, cost reduction is limited. Furthermore, in applications in which the flow condition of underground water is visualized based on a change and transition in underground electrical characteristics, rapidly changing phenomena cannot be captured. This results in limiting the scope and range of applications.
Studies have been reported which attempted to reduce the measurement time by simultaneous transmission from multiple points (Non-Patent Documents 2 and 3). However, the studies relate to a technique of a frequency divide multiple access (FDMA) type which uses different frequencies for the respective transmission points.
The underground electrical characteristics include different frequency response and vary with frequency. Thus, current transmission is desirably provided at the same frequency. However, for the FDMA type, the frequency essentially varies with the transmission point, thus precluding the response potentials from being measured at the same frequency. Furthermore, to allow an increase in the number of simultaneous transmission points, a wider range of transmission frequencies needs to be assigned to the transmission points. Thus, the number of simultaneous transmission points is limited. Moreover, the measurement of the response potentials requires a synchronous detection circuit, thus complicating the apparatus.
Under these circumstances, an efficient measurement method of electrical resistivity profiling has been required, which can provide simultaneous transmission at the same frequency.
Patent Document 1: Japanese Patent Laid-Open No. 2001-021662
Patent Document 2: Japanese Patent Laid-Open No. 2005-337746
Non-Patent Document 1: “Butsuri-Tansa (Geophysical Exploration)” edited by Society of Exploration Geophysicists of Japan, Chapter 5, pp. 259-291
Non-Patent Document 2: “Development of Multi-transmission Electric Resistivity Tomography System and its Application”, Collection of Papers for the 114th Technical Conference of Society of Exploration Geophysicists of Japan, 2006, pp. 215-218
Non-Patent Document 3: “3D Resistivity Exploration Multi-transmission Tomography System”, Collection of Papers for the 115th Technical Conference of Society of Exploration Geophysicists of Japan, 2006, pp. 273-274.